Bulletin No. 15. 

U. S. DEPARTMENT OF AGRICULTURE. 

DIVISION OF SOILS. 



S.23. 



s 

593 



ELECTRICAL INSTRUMENTS 



DETERMINING THE MOISTURE, TEMPERATURE, 
AND SOLUBLE SALT CONTENT OF SOILS. 



LYMAN J. BRIGGS, 

ASSISTANT CHIEF, DIVISION OF SOILS. 




WASHINGTON: 



O i ) V E R N M i: \ T I'RI .\" T I N < ; F F ICE. 



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Bulletin No. 15. 

U. S. DEPARTMENT OF AGRICULTURE, 
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DIVISION OF SOILS. 



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ELECTRICAL INSTRUMENTS 



DETERMINING THE MOISTURE, TEMPERATURE, 
AND SOLUBLE SALT CONTENT OP SOILS. 



LYMAN J. BRIGGS, 

ASSISTANT CHIEF, DIVISION OF SOILS. 




WASHINGTON 



GOVERNMENT PRINTING OFFICE. 



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UL 2 1907 
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LETTER OF TRANSMITTAL 



U. S. -Department of Agriculture, 

Division of Soils, 
Washington, I). C, March 30, 1899. 
Sir: I have the honor to transmit herewith a report upon electrical 
instruments for determining the moisture, temperature, and soluble 
salt content of soils, prepared by Mr. Lyman J. Briggs, assistant chief, 
and to recommend that it be published as Bulletin No. 15 of this 
division. 

Respectfully, 

Milton Whitney, 

Chief of Division. 
Hon. James Wilson, 

Secretary of Agriculture. 



CONTENTS. 



Page. 

Introduction 7 

Electrical method of moisture determination : 

Soil hygrometer 10 

Bridge wire 11 

Balancing mechanism 11 

Battery switch 12 

Scale 12 

Telephone receiver 11 

Induction coil 11 

Condenser 14 

Current interrupter and battery 15 

Method of operating the soil hygrometer 15 

Location of faults in the instrument 17 

Soil electrodes and compensating temperature cell 19 

Adj ustahle temperature cell 20 

Carbon electrodes 21 

Adj ustable metal electrode 22 

Installing the electrodes and temperature cell 23 

Deep electrodes 23 

Shallow electrodes 24 

Installing cell 24 

Connection of electrodes and cell to soil hygrometer 24 

Wire 24 

Splicing and insulating wires 25 

Adjusting the readings of the hygrometer by means of the temperature 

cell 26 

Electrical method of determining temperature: 

Electrical thermometer 27 

Temperature coils 29 

Use of the electrical thermometer 30 

Location of faults 30 

Adjustment of instrument 31 

Standardization of temperature coils 32 

Apparatus for determining the soluble salt content of soils : 

Electrolytic bridge '-- 32 

Rotary switch 32 

1 isc of the electrolytic bridge 34 

Electrolytic cell 35 

Location of faults 35 

5 



ILLUSTRATIONS. 



Page. 
Fig. 1. Soil hygrometer closed, ready for use, showing binding posts, scale and 

central plunger, operating ha.ttery switch 10 

2. Soil hygrometer open, showing battery, condenser, bridge-wire, and 

hattery switch 11 

3. Sectional view of the soil with electrodes and cell properly connected 

to the hygrometer 16 

4. Diagram of interior connections of hygrometer 18 

5. Longitudinal section of adjustahlo temperature compensating cell 20 

6. Rectangular carhon electrode, showing saw-cut contact with wire 21 

7. Carhon electrode with longitudinal section to show construction 21 

8. Splicing and insulating wires - 25 

9. Resistance-temperature curve for the iron wire used in the tempera- 

ture coils 29 

10. Diagram ol interior connections of electrical thermometer 31 

11. Diagram of interior connections of electrolytic bridge . ■ 33 

12. Electrolytic cell and mercury cups 34 

6 



ELECTRICAL INSTRUMENTS FOR DETERMINING THE MOISTURE. 
TEMPERATURE, AND SOLUBLE SALT CONTENT OF SOILS. 



INTRODUCTION 1 . 

The determination of the water content of soils in place by means 
of the variation in the electrical resistance was first investigated by 
Prof. Milton Whitney in 1887. He first attacked the problem by meas- 
uring the variation in the internal resistance of an earth battery, con- 
sisting - of alternate copper and zinc plates buried in the soil. This 
method was found not to be satisfactory on account of the polarization 
of the plates. Measurements of the resistance between large copper 
plates buried at the desired depth were next made with a direct cur- 
rent, but polarization again interfered seriously. The Kohlrausch 
bridge method, employing alternating currents, was finally successfully 
used, and various forms of electrodes were tested, difficulty being 
experienced at that time in maintaining good contact between the 
electrodes and the soil. 

In 1895 the method was further developed in the Division of Soils by 
Professor Whitney and Dr. F. A. Wolff. The resistance-temperature 
coefficient of the soil, which in the preliminary work had not been con- 
sidered, was determined in order that the measurements might be cor- 
rected for temperature. An instrument for field measurements was 
designed, which could also be used for determining the temperature of 
the soil by measuring the resistance of a small hermetically sealed cell 
containing an electrolyte, the temperature coefficient of which was 
known. 

In 1890 the writer eliminated the effect of temperature on the meas- 
urements by balancing the soil resistance against an electrolyte hav- 
ing the same temperature coefficient as the soil, placed in a sealed cell 
near the electrodes, so as to acquire the same temperature. The method 
of moisture determination as thus developed, together with methods of 
determining the temperature and soluble salt content, were published 
in 1897 in bulletins jSTos. 6, 7, 8, and 12 of the Division of Soils. 

It is the object of this bulletin to describe the instruments and meth- 
ods at present employed by this division in investigating the moisture 
and temperature of soils in the field, together with a convenient field 
apparatus for investigating the soluble salt content of soils. Several 



important modifications in the instruments and methods, as previously 
described in other bulletins of the division, have been made. A special 
instrument is now used for each of the three classes of determinations, 
instead of a single instrument as heretofore. This change greatly sim- 
plifies the instruments, makes them easier to operate, materially lessens 
their cost, and, in the case of the moisture and temperature instru- 
ments, permits the use of direct reading scales, thus avoiding, except 
in cases where more than ordinary accuracy is desired, the necessity of 
any reduction of the results obtained. 

The necessary information regarding the water content of the soil can 
of course be obtained by the ordinary method of sampling and deter- 
mining the loss in weight through drying, and when carefully done 
probably no more accurate method exists. It was for the purpose of 
avoiding the laborious sampling, weighing, and drying, with certain 
errors commonly incident thereto, and for the advantage of being able 
to make determinations frequently and quickly in the field, that the 
electrical method of moisture determination was devised. While 
equally valuable experimentally, this method is also capable of certain 
economic applications. 

It is believed that an important use of the moisture apparatus 
is to be found in connection with irrigation operations, in determining 
when there is sufficient water present in the soil and when it becomes 
necessary to add more water. It is obvious that a knowledge of the 
water content of the soil at any desired distance below the surface, 
even though it be only approximate, must prove to be of great service 
in the intelligent application of water. With a knowledge of the 
water content of the soil at various depths, and an understanding of 
the peculiarities of the crop grown, water can generally be supplied 
only as needed and all excess of water, resulting in seepage with the 
attendant translocation of salts which has proved to be so injurious, 
can probably be largely avoided. This excess of water can thus be 
saved to be applied in a useful way, and the deej>er rooting of the 
plants, resulting from the more judicious application of water, will 
render them less sensible to conditions approaching drought. 

The moisture instrument is also generally well adapted to indicate 
the water content of the beds of commercial greenhouses and has been 
used successfully in this connection in some large violet houses in the 
suburbs of Washington. Since the water content of the soil, however, 
is only one of the several factors which must be considered in success- 
ful greenhouse management, the indications of the instrument as apply- 
ing to the treatment of the plant must always be considered in connec- 
tion with the temperature, humidity, and amount of sunshine. When 
used in this manner it is believed that the apparatus may prove to be 
of much service in the commercial greenhouse. 

Used as above mentioned little more is required of the moisture 
apparatus than to indicate when certain limits representing drought 
and excess of water, respectively, have been reached, and to show at 



any time the state of the water content with, reference to these limits. 
In other words, it is a knowledge of the departure of the water content 
from the optimum condition rather than the absolute percentage of 
water that is desired. In order, however, that the percentage of water 
may be obtained when desired, the instrument is provided with a special 
scale from which the approximate water content, expressed in percent- 
age of the dry weight of the soil, may be determined directly, after a 
single standardization to give the relation between the readings of the 
instrument and the actual water content has been made by drying sam- 
ples of the soil. Since the relation betweeu the electrical resistance 
and the water content is not strictly the same for all soils, in case espe- 
cially accurate moisture determinations are required, as in the com- 
parison of cultural methods, it is advisable to make careful moisture 
determinations from time to time by sampling and drying, in order to 
determine the corrections, if any. to be applied to the instrument 
readings at different parts of the scale for the particular soil under 
investigation. 

ELECTRICAL METHOD OF MOISTURE DETERMINATION. 

The electrical method of moisture determination is based upon the 
principle that the resistance offered to the passage of au electrical 
current from one electrode to another buried in the soil, varies with 
the amount of water present in the soil. In nearly all soils throughout 
the range of water content favorable to plant development, it has 
been found by Whitney and Gardner 1 that the electrical resistance is 
very nearly inversely proportional to the square of the water content 
expressed in percentage of the dry weight of the soil. 

The soluble salts of the soil form with the moisture a salt solution, 
and it is the amount, concentration, and temperature of this salt solu- 
tion that determines the resistance of the soil. In order to employ the 
electrical resistance as a means of measuring the water content, it is 
necessary to maintain the temperature and the amount of salt in solu- 
tion constant, or to correct for their variation. Changes in resistance 
due to temperature are corrected by balancing the resistance betweeu 
the soil electrodes against the resistance of an electrolytic cell, which 
is buried near the soil electrodes and which is tilled with a salt solution 
having the same temperature coefficient as the soil itself. As regards 
the quantity of soluble salts present, it can be said that no gradual 
variation in the resistance due to a change in the salt content seems 
to take place during a seasou. In a few instances a sudden change in 
the position of the moisture curve, following very heavy rains, indi- 
cated that a movement of the soil or a leaching of salts had taken 
place. Such a change, however, will at once be apparent and can be 
easily corrected by restandardizing the instrument. 

The electrodes are buried in the soil at the desired depth at the 

1 Bulletin No. 12. Division of Sails, p. 15. 



10 

beginning of the growing season and remain undisturbed during the 
development of the plant. From these electrodes insulated wires lead 
to the measuring instrument, which may be located at any convenient 
place. By this method of moisture determination one is enabled to 
investigate the changes in water content in the same portion of soil 
throughout the season, instead of dealing with different samples, as is 
necessitated in the tube method of sampling and drying. The advan- 
tage of always working with the same portion of soil is emphasized by 
the fact that a difference in water content of 2 or 3 per cent in dupli- 
cate samples is not an uncommon experience with those who have 
used the tube method. 

SOIL HYGROMETER. 

The moisture instrument, by means of which the resistance between 
the soil electrodes is measured and to which the name "soil hygrome- 




Fi«. 1. — Soil hygrometer closed, ready for use, showing binding posts, soalt 

operating battery switch. 



and central plunger 



ter" has been applied, is an adaptation of the well known Wheatstone 
bridge method of measuring electrical resistances. The instrument, 
which is illustrated in figs. 1 and 2, consists of a slide-wire bridge, 
provided with an induction coil, current interrupter, battery, and tele- 
phone receiver, suitably arranged for electrolytic measurements and 
inclosed in a small wooden case 8 inches long, 7 inches wide, and 4| 
inches high. The inside of the case is partitioned off in such a man- 
ner as to provide a space for the battery and also to form a small box 
for the reception of the current interrupter and induction coil. In the 
top of a partition running lengthwise through the middle of the box a 
series of steps is cut, on which the two springs of the battery switch 
are fastened in such a manner as to bring the ends of the springs 
directly beneath a plunger in the balancing mechanism. The central 
partition carrying the battery switch, together with the small case 
inclosing the current interrupter, can be seen in fig. 2. 



11 



BRIDGE MIRE. 

The bridge wire consists of a No. 24 B. and S. platinoid wire about 
64 cm. long'. This wire is mounted upon the periphery of a wooden disk 
on the inside of the cover of the box in such a manner as to permit 
contact with a movable slide. The wire on the disk is so adjusted that 
there remains free at one end a portion about 16 cm. long, and at the 
other end a portion about 8.5 cm. in length. These are so coiled as to 
prevent short circuiting and are carefully soldered at their free ends to 
the left and right hand binding posts, respectively, as shown in the illus- 
tration. The wooden disk which carries the bridge wire is constructed 
of well-seasoned cherry and thoroughly covered with shellac. The 
periphery of this disk is smoothly polished and has a slight groove cut 




Fig. 2.- 



-Soil hygrometer open, showing battery, screw for adjusting condenser, bridge win 
battery switch. 



and 



in its surface one-fourth inch from the exposed face, the groove being of 
sufficient depth to retain the bridge wire in place and yet not interfere 
with the action of the slider in making contact with the wire. The 
bridge wire is stretched tightly in the groove and secured to the disk 
by soldering to two small pins driven radially into the disk about one- 
half inch apart. 

BALANCING MECHANISM. 

The balancing of the bridge is secured by the rotation of a shaft, 
working through a bushing inserted through the cover of the box and 
the bridge-wire disk, the whole being coaxial with the periphery of the 
disk. The bushing is retained in place by a brass collar, which is 



12 

soldered to its upper end and secured to the cover of the box. This 
collar also assists in keeping in place the celluloid cover of the paper 
scale. 

From the lower end of the shaft, passing through the cover of the 
box and the bridge- wire disk, extends an arm three-eighths inch wide, 
parallel to the face of the wooden disk supporting the bridge wire. 
Just beyond the periphery of the disk the arm bends upward at right 
angles and carries the bridge- wire contact spring, which consists of a 
piece of No. 32 B. and S. spring brass one-eighth inch wide, doubled 
back upon itself in such a manner as to bring the contact under the 
arm, thus protecting the contact from mechanical injury. 

The shaft is held in position in the bushing by means of a collar 
which carries the scale pointer, and is secured by a set-screw. The 
collar thus furnishes means of adjusting both the shaft and the pointer. 
Through the center of the shaft of the balancing mechanism there 
operates a plunger for closing the battery circuit. This plunger is 
keyed in such a manner as to prevent rotation within the shaft, while 
at the same time moving freely in the line of its axis. To the upper 
end of the plunger is fastened a circular handle of hard rubber which 
serves both to operate the plunger and rotate the balancing mechanism. 
The lower end of the plunger is cut down for a short distance to a 
smaller diameter, which works through a corresponding hole in the 
lower part of the shaft. A weak coiled spring, just sufficient to raise 
the plunger when released, is placed between the shoulder of the 
plunger and the bottom of the shaft. To the lower end of the plunger 
a small disk of hard rubber is fastened by means of a countersunk 
screw. This retains the plunger in place and also serves to insulate it 
from the battery switch. 

BATTERY SWITCH. 

The battery switch consists of two platinum-tipped springs so 
mounted upon the partition running through the center of the box 
that when the plunger is depressed the upper spring is forced down 
against the lower one, thus closing the battery circuit. This switch 
has the advantage of a rubbing contact, since the plunger has a move- 
ment sufficiently great to depress the lower spring for a short distance, 
thus causing a rubbing movement between the two springs and insur- 
ing a perfect contact. The switch is also highly advantageous from 
the fact that it thoroughly protects the battery of the instrument, 
since the moment the hand is taken from the handle of the balancing 
mechanism the plunger is released and the circuit is opened. Thus the 
battery is used oidy during the actual observation, and consequently 
has a much longer life. 

SCALE. 

The soil hygrometer is provided with a direct reading scale, the read- 
ings of the instrument being expressed in terms of the percentage of 
some definite water content, taken as a standard. In order that this 
may be accomplished it is necessary at the time the instrument is 



13 



installed to adjust either the electrodes or the temperature cell so that 
the reading - of the instrument may correspond with the water content of 
the soil at that time. The manner in which this is accomplished will 
be considered later. The direct reading scale is based upon the fact 
that within the limits between which moisture observations are usually 
made the electrical resistance of the soil is inversely proportional to 
the square of the water content, expressed in percentage of the dry 
weight of the soil. 1 This relation does not hold strictly for all soils as 
we approach the limits, but the agreement is so close that for ordinary 
observations it was not deemed necessary to provide the instrument 
with special scales for different soils. 

If we take 100 on the scale to express some particular percentage of 
water, which should be chosen to agree as nearly as possible with the 
most favorable water content of the soil, then 75 expresses the condi- 
tion of the soil when it contains but 75 per cent of the optimum water 
content, and 125 expresses the condition when it contains 125 per cent 
of the amount present in the optimum condition, and so on. The mid- 
dle point of the bridge wire is chosen as the 75 point of the scale. If 
we assume the resistance at this water content to be 1,000 ohms, for 
convenience in computation, the corresponding resistance for any other 
point of the scale can be calculated from the following formula: 



A\ 



75" 

-"75 — 2 



in which x represents the point on the scale, the corresponding resist- 
ance of which we wish to determine, E- 5 represents the resistance chosen 
for the 75 point, in this case 1,000, and R K represents the required 
resistance corresponding to the point x. For this formula the follow- 
ing calibration table has been prepared. 

Hygrometer calibration table. 



Scale 


Resistance 


X. 


fix. 


140 


287 


135 


309 


13U 


333 


125 


360 


120 


391 


i 115 


425 


110 


465 


105 


510 


100 


563 


95 


623 


90 


694 


80 


879 


85 


779 


80 


879 


75 


1,000 


70 


1, 14s 


65 


1,331 


60 


l. 56:; 


55 


1,860 


50 


2, 250 


45 


2,778 


40 


3,516 


35 


4,592 


30 


6. 252 



Bulletin No. 12, Division of Soils. 



14 

To calibrate the hygrometer a resistance of 1,000 ohms is inserted 
between the middle and left-hand binding posts. The resistances 
given in the table are then inserted successively between the middle 
and right-hand binding posts. When the bridge is balanced against 
any of these resistances the table gives the corresponding scale reading. 

To avoid error, due to lack of uniformity in the bridge wire, the 
scale is calibrated for every 10, or better, for every 5 divisions. The 
bridge wire is chosen of such a length that the graduation extends 
through the whole available space on the scale, so as to make the scale 
as open as possible. The scale is protected by a sheet of transparent 
celluloid, the edges of which are held in place by a nickel-plated ring 
secured to the top of the box, as shown in the illustration. 

TELEPHONE RECEIVER. 

The telephone receiver used in balancing the bridge is of the watch- 
receiver form, with a hard-rubber case to minimize the danger of a 
stray circuit from the receiver to the earth. One terminal of the 
receiver is connected with the balancing mechanism by means of a 
spring brass friction contact between the bridge-wire disk and the 
outer bushing of the balancing mechanism. The other terminal is con- 
nected to the middle binding post of the instrument. 

INDUCTION COIL. 

In order to prevent polarization of the soil electrodes it is necessary 
to employ an alternating current for measuring purposes. This is 
secured by the use of a small induction coil, 2f inches in length, with 
a laminated core about one-half inch in diameter, built up of No. 15 
B. and S. soft iron wire. The primary of the induction coil consists of 
four layers of No. 24 B. and S. D. 0. O. copper wire and the secondary of 
three layers of No. 36 copper wire. This coil is placed in the bottom 
of the small case which holds the current interrupter, and is held in 
position by means of cork wedges placed at the ends. The terminals 
of the secondary coil are soldered to the side hinges. These hinges are 
in turn connected to the right and left hand binding posts. The hinges 
thus form very convenient flexible contacts, combining both compact- 
ness and durability. 

CONDENSER. 

Owing to the fact that a small but appreciable capacity may exist 
between the two electrodes buried in the soil, it has been necessary, in 
order to secure sharp readings, to place a small condenser parallel with 
that arm of the bridge adjacent to the soil resistance, which would 
throw the two capacities on opposite sides of the bridge with respect 
to the telephone receiver (see fig. 4). In this way the disturbing 
capacity in the soil can be entirely eliminated, and the minimum in the 
receiver becomes sharp and distinct. Since this capacity between the 



15 

electrodes in the soil varies with different moisture contents of the soil, 
and under certain other conditions which have not yet been fully deter- 
mined, it is necessary that the condenser be capable of adjustment. 
A small condenser has accordingly been devised which is capable of 
continuous adjustment throughout the range of capacity necessary to 
balance the capacity effect between the electrodes in the soil. This 
condenser depends upon the principle that the electrostatic capacity of 
a series of insulated plates varies greatly with the distance between 
the plates, the capacity being inversely proportional to the square of 
this distance. Spring brass strips, 2 inches wide and 3 inches in length, 
are given a slight curvature, so that wheu the plates are stacked up 
between thin sheets of mica with the alternate plates projecting at 
opposite ends of the pile the plates and mica do not form one compact 
mass, but are separated to a greater or less degree, depending upon the 
curvature of the plates and their thickness. To the ends of the plates 
projecting from one end of the pile is soldered a common terminal, while 
a similar terminal is soldered to the plates at the other end. These two 
sets of plates are in this way thoroughly insulated from each other by 
means of the pieces of mica, while the distance between them can be 
greatly varied by simply compressing the pile. A small hard rubber 
plate is placed upon the top of the condenser as thus built up, and the 
whole is placed in a frame which has a small compression screw, the end 
of which works in a corresponding depression in the center of the hard- 
rubber plate. Upon changing the position of this compression screw 
we compress or release the condenser, and so vary its capacity at will. 
The terminals of the condenser are connected by means of the hinges 
with the middle and right-hand binding posts. 

CURRENT INTERRUPTER AND BATTERY. 

The current interrupter consists of a small pocket buzzer giving 
about 120 interruptions per second. This buzzer is placed in a small 
case and packed with cotton wool, which serves to inufrle all sound 
from the buzzer and thus facilitates the discernment of slight sounds 
in the receiver. This form of interrupter is of great convenience in 
such an instrument, as it is protected from injury and permits rough 
treatment without requiring readjustment. The case for the battery is 
of such dimensions as to allow any dry cell of standard size to be used. 
This enables one to secure without difficulty a dry cell adapted for use 
in the instrument when it becomes necessary to supply a new cell. 
The battery is held in its case by means of cork wedges, which permit 
it to be easily and quickly removed. 

METHOD OF OPERATING THE SOIL HYGROMETER. 

In using the soil hygrometer in the held it is customary to provide a 
small platform, consisting of a stake with a board nailed across its top, 
upon which to rest the instrument. The arrangement of the connec- 
tions is illustrated in tig. 3. The wire leading from the temperature 



16 

cell is connected to the left-hand binding post of the instrument; the 
wire which is common to both cell and electrode is attached to the 
middle binding post, while the third wire is attached to the right-hand 
binding post. The ends of the wires should be scraped bright with a 
dull knife, in order to insure good electrical connections with the bind- 
ing posts. 

These connections being made, the telephone receiver is pressed 
tightly against the ear and the handle of the instrument pushed down, 
when a buzzing sound will be heard in the receiver. Holding the 




.fi'iG. 3. — Sectional view of the soil, with electrodes and cell properly connected to the hygrometer. 

handle down so as to keep the battery switch closed, the pointer is 
rotated to either right or left until the position is found at which the 
note in the telephone receiver is no longer heard. On rotating the 
pointer to either side of this position the sound in the receiver should 
gradually increase. In case difficulty is found in locating the exact 
position of balance, it will be found to be of assistance to rotate the 
pointer rapidly back and forth over the position of no sound, locating 
points of equal sound intensity on either side. The mean position 



17 

between these two points gives the position of balanee, and the number 
opposite the pointer gives the desired reading. 

In case it is not possible to obtain a well-defined minimum in the 
sound in the receiver, recourse should be had to an .adjustment of the 
screw of the condenser inside the box. Upon compressing or releasing 
the condenser a position of the compression screw will be found at 
which the minimum in the receiver is much more distinct. In making 
the preliminary readings with the instrument it will usually be found 
more satisfactory to release the condenser as much as possible by 
loosening the compression screw. If a number of electrodes are to be 
read by the same instrument, it may be found necessary to adjust the 
condenser for the various electrodes. Usually, however, a common 
adjustment of the condenser can be found which will be satisfactory for 
all readings. 

If the instrument is to be shipped, it is advisable to compress the 
condenser in order to prevent possible disarrangement of the condenser 
plates. It is also necessary in such cases to place a washer between 
the head of the plunger and the end of the shaft, or else disconnect 
one terminal of the battery, in order to prevent the closing of the 
battery circuit during transportation. 

LOCATION OF FAULTS IN THE INSTRUMENT. 

It may sometimes happen that during a journey or through careless 
handling an instrument may get out of adjustment or fail to work 
properly. Some of the faults which are likely to occur will conse- 
quently be discussed. 

In case there is no sound in the telephone receiver when the battery 
switch is closed, the failure may be due to (1) a run-down battery; (2) 
lack of contact between the two springs of the battery switch, due to 
dirt on the platinum contacts; (3) improper adjustment of the current 
interrupter; (4) broken connections, either in the primary or secondary 
circuits of the induction coil; (5) failure of the contact spring of the 
balancing mechanism to make contact with the bridge wire; (G) broken 
receiver circuit. 

The question as to whether the difficulty exists in the bridge connec- 
tions or is duo to the current interrupter can generally be decided by 
closing the battery switch and placing the ear close to the current- 
interrupter box, when, if the interrupter is working, a slight note will 
be heard. If the interrupter does not work, it should be first examined. 
This is done by taking the cover off from the small case containing the 
interrupter, which will be found packed in cotton. The current inter- 
rupter should be taken out and the metal cover removed. Keeping 
the battery switch closed, it should now be determined whether the 
interrupter can be made to work by some simple adjustment of the 
contact. If this can not be done, the connections with the battery and 
induction coil should be examined. If these connections are found to 
be perfect, it is probable that the battery is defective and should be 
replaced by ;t new one. 

19805— No. 15 L> 



18 



In case the current interrupter works satisfactorily and still no sound 
can be beard in the receiver, it is evident that a broken circuit exists 
This can generally be found without difficulty by carefully examining 
the connections, which should be in accordance with the accompanying 
diagram (fig. 4). 

If the difficulty appears to be in the bridge connections, the bridge- 
wire slider should be first examined, to see whether it makes contact 



^\0GE_JV/^ 



BRIDGE WIRE 
CONTACT 



TO 

TELEPHONE 
RECEIVER 



BATTERY 




BATTERY SWITCH 



Fig. 4.— Diagram of interior connections of hygrometer 



with the bridge wire, and adjusted, if necessary, by carefully bending 
the contact spring up or down until the slider makes contact for all 
positions of the scale. This is the cause of the trouble when a note 
can be heard in the receiver for certain parts of the scale only. In case 
the bridge wire has become loosened, it should be unsoldered from one 
of the pins in the periphery of the disk, carefully placed in the groove 
in the disk, drawn taut, and resoldered. 



! 



19 

In case the faults seem to be in the receiver circuit, the connections 
inside the box should be examined, as well as the screws binding the 
cord terminals to the telephone receiver. If necessary, the face of the 
receiver can be removed by unscrewing it and the inside connections 
examined. 

A pair of test coils accompany each instrument. These coils are to be 
connected Avith the three binding posts in such a manner that the com- 
mon terminal of the two is connected to the middle post and the single 
terminals to the right and left hand binding posts. When these two 
coils are connected to the instrument in the manner described, a balance 
of the bridge should be obtained at the 75 point of the scale. In case 
the balance can not be obtained at or near this point, the instrument is 
either oat of adjustment or else broken connections or short-circuiting 
exists. If no balance of the bridge can be obtained by means of this 
coil, all of the connections should be carefully examined to see if they 
correspond with those given in the diagram (fig. 4). The trouble might 
also be due to a short-circuiting in the condenser, which would connect 
directly the middle and left-hand binding posts. In such cases the 
connection of the condenser with the middle hinge should be broken 
in order to see if this remedies the trouble. 

If a balance is obtained at a position differing by two or three divi- 
sions from the 75-poiut mark, the instrument is out of adjustment. This 
can be remedied by loosening the screw in the pointer collar, and, hold- 
ing the contact arm rigidly at the position of balance, bringing the pointer 
to coincide with the 75-ppint position, and then tightening the screw in 
the collar. In case it should be necessary to remove the shaft of the 
balancing mechanism for the purpose of repairing or renewing the plati- 
num bridge-wire contact, this adjustment should always be made after 
the instrument is once more assembled. 

The adjustment of the instrument should always be determined by 
the use of these test coils before beginning field work. A failure to 
secure a balance of the instrument when connected with the soil elec- 
trodes in the field, provided the instrument is already in adjustment, 
will be discussed later (p. 2G). 

SOIL ELECTRODES AND COMPENSATING TEMPERATURE CELL. 

It has been mentioned above that the effect of varying temperature 
on the soil resistance is compensated by balancing this resistance against 
the resistance of an electrolytic cell containing a solution at the same 
temperature and having the same temperature coefficient as the soil. 
The resistance of different soils, however, varies so greatly that in order 
to bring the readings upon the most sensitive portion of the instrument 
scale it is necessary that the ratio of the soil resistance to that of the 
temperature cell be capable of adjustment. This can be accomplished 
by varying the resistance of the compensating cell or by changing the 
area of the soil electrodes. Both methods have been employed, but 
the former is more convenient and is the one generally recommended. 



20 



ADJUSTABLE TEMPERATURE CELL. 

The adjustment of the instrument reading by changing the resistance' 
of the temperature cell instead of changing the area of the soil elec- 
trodes lias the important advantage of obviating the necessity of dis- 
turbing the electrodes. This also permits the use of carbon instead of 
metal electrodes, the former seeming to possess some advantages over 
over the latter. 
The adjustable temperature cell is illustrated in tig. 5, which, shows 
a longitudinal section. It is constructed of brass 
and hard rubber, and is made in two parts, one of 
which is capable of adjustment within the other. 
The cell is used in a vertical position, the cylindrical 
bulb being at the top. The lower part of the outer 
portion consists of a piece of hard-rubber tubing, 3 
inches long and five-sixteenths inch inside diameter, 
with walls one sixteenth inch thick. Into the lower 
end an unburnished, nickel-plated, brass plug is 
sealed, which forms one electrode. The brass cyl- 
inder, nickel-plated on the inside, constitutes the 
other electrode. Suitable insulated connecting 
wires are soldered to the two electrodes and secured 
to the sides of the cell with wax. The outer surfaces 
of both electrodes are insulated by thoroughly cov- 
ering them with insulating wax or paint. The ad- 
justable plunger consists of a hard-rubber rod, 5 
inches long and one-fourth inch in diameter, which 
works through a short piece of closely fitting flex- 
ible rubber tubing attached to the tubulure on the 
end of the brass cylinder. 

The cell is filled to a point one-fourth inch above 
the upper end of the hard-rubber tubes with a salt 
solution which has the same electrical temperature 
coefficient as the soil. The solution consists of nine 
parts of a solution of four-fifths normal sodium chlo- 
ride and one part of 95 per cent alcohol. The deter- 
mination of the resistance-temperature coefficient of 
soils, which gave the data for the preparation of this 
solution used in the temperature cell, has been 
described in Bulletin No. 7 of this division. 

When the plunger of the adjustable cell is pushed 
down into the solution, a portion of the solution is displaced and the 
resistance of the cell is increased. By thus varying the position of this 
plunger we are enabled to increase continuously the resistance to about 
three times the resistance of the cell when the inner tube is drawn down 
entirely out of the lower portion of the cell. In adjusting the reading 
of the instrument the plunger is moved up or down until the desired 
reading is obtained. This form of compensating cell has sufficient 



Fig. 5. — Longitudinal sec- 
tion of adjustable tem- 
perature compensating 
cell. 



21 

range to permit adjustment in all cases except when the electrodes are 
much too large or too small. In such cases it is necessary to increase 
or diminish the size of the electrodes and then make the final adjust- 
ment by means of the cell as above. 



CARBON ELECTRODES. 



Carbon forms a more satisfactory electrode than metal, since it seems 
to give a more perfect contact with the soil grains, due undoubtedly 
to the fact that the moisture permeates to some extent 
tlie carbon itself. 

A form of electrode which has been used extensively 
in connection with shallow depths consists of a rectan- 
gular strip of carbon 3 inches long, five eighths of an 
inch wide, and three-sixteenths of an inch thick. A 
series of diagonal cuts (see fig. 6) are made in the edge 
of the carbon, just, wide enough to allow the connecting 
wire to be forced into place, the contact being after- 
wards covered with insulating wax. A more substan- 
tial contact is made by boring a hole through the carbon 
strip near one end, in which is inserted an uulacquered, 
round headed, brass machine screw, provided with a 
nut. A finch 8-32 machine screw is suitable for this 
purpose, the hole beiug of such a size as to make a 

snug fit with the screw, which 
is drawn tight by means of the 
nut. The connecting wire is 
then soldered with rosin in 
the slot of the screw-head and 
allexposed metal covered with 
insulating paint. 

A form of electrode which 
is well adapted to both deep 
and shallow work is illus- 
trated in fig. 7, together with 
a longitudinal section, show- 
ing the manner in which con- 
tact with the wire is made. 
The electrode is one-half of an inch in diam- 
eter and 2£ inches long, the general form 
being that of a paraboloid of revolution. 

This form is given so that when the electrode 
is forced into a conical hole in the soil perfect 
contact will be made at every part of the 
electrode. A cylindrical cavity with corrugated sides is made in the 
upper end of the electrode, into which the end of the connecting wire 
is inserted, and which is then nearly filled with fusible metal which 
holds the connecting wire firmly in place and makes perfect contact 




Fig. (i. — Rectangu- 
lar carbon elec- 
trode, ahowiug 
saw -cut contact 
with wire. 



Pig; 7.— Carbon electrode, with 
longitudinal section to show con- 
struction. 



22 

with the carbon. The upper end of the electrode arid any exposed por- 
tion of the conducting wire are covered with insulating wax. 

A modification of the electrode just described cau be constructed in 
the laboratory from unplated dense electric-light carbons (preferably 
those made for inclosed arc lamps). These carbons, which are one-half 
of an inch in diameter, are sawed into 3-inch lengths and ground to a 
conical point on a stone. A three-sixteenths inch hole is then drilled 
in the other end to the depth of half an inch, which is afterwards cor 
rugated by means of a suitable turning tool. The end of the insulated 
wire, which has been scraped bright and clean, is bent into a loop and 
inserted in the corrugated cavity, around which is run a fusible metal. 
Wood's fusible metal, consisting of 1 to 2 parts of cadmium, 2 parts of 
tin, 4 parts of lead, and 7 to 8 parts of bismuth, which melts at 60° to 
70° C, has been found suitable. It is advisable to heat the carbon 
about the cavity with a Bunsen flame before running in the metal, 
so that the latter will not be cooled by coming into contact with the 
carbon. The metal and wire should finally be thoroughly covered 
with insulating wax as before. 

ADJUSTABLE METAL ELECTRODE. 

When the adjustment is made by changing the area of the electrode, 
it is most convenient to use metal electrodes of nickel-plated copper 
wire of about No. 24 B. and S. gauge. Two of these wires, 4 or 5 feet 
in length, are attached at one end to two insulated connecting wires 
by means of the telegraph splice (see p. 25), the conducting wires 
being of sufficient length to reach from the point where the electrodes 
are to be buried to where the instrument is located. The splice is now 
thoroughly insulated with wax and tape (see p. 25), and the wires are 
buried at the desired depth, parallel to each other and from 1 to 2 feet 
apart, by excavating two very narrow ditches in the soil, after which 
the soil is thoroughly tamped back into place. When the ground 
about the electrode has become thoroughly settled, which generally 
requires two or three days, a small excavation is made in the soil at the 
free end of the wire, and small portions of the electrode are cut off 
until the desired reading is obtained. It is, of course, necessary to 
have the electrode entirely covered at the time the observation is 
made. This method of adjustment is not as convenient as that of the 
adjustable temperature cell, and is open to the rather serious objec- 
tion that the electrodes are not in a normal condition and are liable to 
undergo slight subsequent changes. 

If the investigation of the water content of a certain layer of the 
soil is desired, the wire is bent into a zigzag form of such dimensions 
as to traverse the depth of the layer under investigation. When this 
form of electrode is desired it is" not necessary to use the adjustable 
temperature cell, and it has been customary to use a form which is her- 
metically sealed, although the adjustable cell is equally well adapted to 
this use, and possesses the advantage that the final adjustment can be 
made by the cell instead of by means of the electrodes. 



23 



INSTALLING THE ELECTRODES AND TEMPERATURE CELL. 

In the form of moisture instrument previously used by tliis division 
a shelter box was placed near the plats of ground on which moisture 
investigations were being made, and the various electrodes and tem- 
perature cells were connected by underground lead-covered cables with 
these shelter boxes in which the moisture instruments were kept. The 
terminals of the various electrodes and temperature cells were brought 
up to a suitable switch board, which enabled connections to be quickly 
made with the moisture instrument. This arrangement is somewhat 
expensive, however, especially in case the different plats under investi- 
gation are some distance apart. It is therefore recommended that the 
three wires leading from the electrodes and the temperature cell be 
brought to the surface a short distance from the electrodes, and that 
the soil hygrometer be carried from point to point, and connected with 
these wires at the time of taking the observations. 

In installing the electrodes for moisture observations it is of course 
important that a typical portion of land be selected, special precaution 
being taken to select ground which is not depressed at that point, as 
any depression would tend to permit an excessive accumulation of 
water, and thus render the moisture observations inaccurate when 
applied to the conditions over the whole plot. 

It is also important that the electrodes should be located in such a 
position as not to be disturbed by cultivation. If the electrodes are 
less than G inches deep, the treading of a man or a horse on the ground 
immediately above the electrodes might so alter the compactness of 
the soil and its relation to water as to change materially the readings 
of the instrument. In such cases, therefore, both the electrodes and 
temperature cell must be so placed as to be out of reach of the culti- 
vator and the horse, and care must be taken not to tread on the earth 
immediately above the electrodes when taking the observations. At 
lower depths such precaution is not necessary. For most held crops 
in Eastern soils the water content at a depth of 5 to 8 inches seems to 
give the most important information regardiug the moisture content of 
the soil. The depths at which moisture observations should be carried 
must necessarily vary to some extent with different soils and under 
different conditions, and the particular depths at which the electrodes 
should be placed will suggest themselves to the investigator. 

DEEP ELECTRODES. 

In installing the electrodes at the lower depths an inch auger hole is 
made in the soil down to the upper limit of the depth at which elec- 
trodes are to be placed. A stick, the end of which is in the form of a 
cone, three-eighths inch in diameter and 2 inches long, is then forced 
down until the base of the cone is even with the bottom of the original 
auger hole. The electrode is then carefully lowered by means of its 
connecting wires into this cavity prepared for it, and pushed firmly 



24 

into place with the other end of the stick. The hole is then filled with 
moist soil, care being taken to thoroughly tamp the soil with the rod 
during the process of refilling. The other electrode is buried in a 
similar way at a distance of 2 or 3 feet from the first. The resistance 
between the electrodes is practically confined to volumes of soil not 
exceeding G inches in diameter, with the electrodes as centers, so that 
there is no special advantage in separating them further, unless it is to 
avoid the effect of a possible lack of uniformity in the soil. 

SHALLOW ELECTRODES. 

In case the electrodes are not to be located deeper than the land was 
plowed the hole can generally be made by the stick alone. The elec- 
trode is then forced into place by means of the stick, and the hole filled 
with earth and thoroughly tamped as before. 

INSTALLING CELL. 

The adjustable temperature cell is filled with the salt solution (see 
p. 20) and buried in the soil somewhat to one side of a line joining the 
two soil electrodes and at a depth such that the lower part of the cell 
occupies the same layer of soil as the electrodes. This is conveniently 
done by boring a hole similar to that made for the electrodes in which 
the cell is placed, any vacant space being carefully filled with soil. To 
adjust the cell a small excavation is made above the cell to expose the 
plunger. In case the cell is located so deep that such an excavation is 
not practicable the plunger may be spliced to a stiff wire (No. 10 iron 
wire is satisfactory) which is cemented in the hole in the end of the 
plunger by means of the insulating wax. The wax should be allowed 
to become thoroughly hardened before attempting the adjustment. 

CONNECTION OF ELECTRODES AND CELL TO SOIL HYGROMETER. 

The wire leading from the upper electrode of the temperature cell is 
spliced (see p. 25)' to the wire leading from one of the soil electrodes, 
the common terminal, which is joined to the middle binding post of 
the soil hygrometer, being known as the combination wire. The other 
wire from the cell is known as the cell wire, and is joined to the left- 
hand binding' post, while the wire from the other carbon is connected 
with the right-hand binding post. (See fig. 4.) 



The wires used for connection should in all cases be provided with 
that grade of insulation known to the trade as " waterproof," either 
when used in the soil or when exposed to the weather. Where wires 
are protected from rain or excessive dampness a cheaper grade of in- 
sulation may be used. In making long connections a three-conductor 
lead-covered c?Jble, which cau be safely buried under ground, is con- 
venient and not expensive. The three conductors of this cable should 
be provided with insulation of different colors to facilitate connections. 



25 

SPLICING AND INSULATING WIRES. 

Care must be exercised in splicing - wires. Tlie best form of' connec- 
tion for field use is the telegraph splice, illustrated in fig. 8, a.^ In mak- 
ing this connection che insulation should be removed from each wire 
for a distance of at least 3 inches from the end and the wire scraped 
bright and clean. The wires are then brought together, their ends 
pointing in opposite directions and twisted together at a point about 
three-fourths of an inch from where the insulation begins. This forms 
the middle part of the splice. Grasping this firmly in a pair of pliers, 
the free end of each wire is wrapped tightly around the straight por- 
tion of the other, as shown. This splice should be used in all cases 
where it is necessary to lengthen wires. In temperature work these 
wires should be soldered if possible after being spliced. 

The method of joining one wire to the middle of another, as when 




Fifi.8. — Splicing and insulating wires: (a) Telegraph splice; (b) Side connection; (c) Taping the Bplice. 

making connection between the temperature cell and the soil electrode, 
is illustrated in fig. 8, b. The insulation is removed for a distance of li 
inches from the wire to which the splice is to be made. The end of the 
other wire is prepared as before. The wires are now tightly twisted 
together close to the insulation for about half an inch, and then the 
end of the free wire is wrapped helically about the other, as shown. 

All splices should be thoroughly insulated, both to prevent the con- 
tact from being impaired through subsequent exposure and to prevent 
the occurreuce of stray circuits. The exposed portions should be first 
covered with insulating wax, which is applied when hot. The prepara- 
tion known as "Chatterton's Compound" has been found very satisfac- 
tory. A thin coating of this will suffice, but care should be taken to 
see that all parts are covered. It is then advisable to wrap the con- 
nection with insulating adhesive tape, which is wound spirally about 
the splice, as shown in fig. 8, c. 



26 



ADJUSTING THE READINGS OF THE HYGROMETER BY MEANS OF THE 

TEMPERATURE CELL. 

After the soil about the electrode has been allowed to settle, which 
usually requires two or three days, and preferably after a heavy rain, 
the wires leading from the electrodes and the temperature cell are con- 
nected with the instrument in the manner described (see p. 24), and a 
reading of the instrument is taken by pushing down the plunger in the 
handle until a note is heard in the telephone receiver, and then rotat- 
ing the handle until a point is found at which the note vanishes. The 
inner rod of the temperature cell is then raised or lowered until the 
desired reading on the instrument is obtained. 

By pushing the inner rod into the temperature cell the reading on 
the instrument becomes higher; conversely, by raising the inner rod 
the instrument gives a lower grading. In case it is not possible to 
secure the balance on the instrument — that is, a point where there is 
no sound in the receiver for any position of the inner rod of the adjust- 
able temperature cell — it becomes necessary either to increase or dimin- 
ish the area of the soil electrodes. If the sound seems to be weaker 
when the inner rod of the compensating cell is pushed entirely down, 
and the pointer is at the lower number, then the soil electrodes are too 
small and must be made larger. If, on the contrary, the sound in the 
receiver diminishes in volume when the pointer approaches the higher 
numbers and the inner rod in the temperature cell stands in its highest 
position, then the electrodes are too large and must be cut down. In 
case no difference can be perceived in either of the positions, it will be 
necessary to change the size of the electrodes and observe the effect. 

In all cases it is desirable to adjust the instrument so that 100 on tbe 
scale represents as nearly as i>ossible the optimum condition as regards 
water content. When the instrument is used in this manner the instru- 
ment readings represent the percentage of the optimum water content 
in the soil. The scale is consequently much superior to purely arbi- 
trary units, since it gives a definite idea of tbe amount of water present. 

In case it is desired to determine the percentage of water in the soil, 
a number of samples for moisture determination should be carefully 
taken at the depth of the electrodes at a time when the soil is in its 
most favorable condition, i. e., when the reading of the instrument is 
near 100. At the same time a careful reading of the instrument should 
be taken. The value of the 100 point of the scale in actual percentage 
of water can be determined from tbe following equation: 

in which P x is the actual percentage of the water as found from the 
samples, which correspond to the reading x on tbe instrument at tbe 
time the samples were taken, while P m is the required percentage of 
the water corresponding to the 100 point on tbe scale. 



27 

To take a concrete case: Suppose that the value of P, as found from 
sampling' and drying was 18.6 per cent, while the reading - x of the 
instrument was 95, we then have 

P m = 18.0 q^ = 19.0 per cent, 

or the water content corresponding to the 100 point on the scale is 19.0 
per cent. 

Having determined the water content for the 100 point, the water 
content corresponding to any other reading can at once be found by 
multiplying the water content at 100 by the scale reading expressed as 
a percentage. To illustrate again : The water content when the instru- 
ment reads 80 in the above example would be 19.0 x .SO = 15.7 per 
cent; when the reading was 125, it would be 19.0 x 1.25 = 23.5 per 
cent, and so on. 

As has been mentioned before, the inverse square law upon which 
the moisture scale was constructed is not strictly applicable to all soils, 
and Avhen especially accurate results are desired, the departure from 
this law for the soil in question should be determined by comparing the 
instrument readings with careful moisture determinations, from which 
the correction to be applied to the instrument readings can be obtained. 
For example, suppose from sampling and drying, the water content 
when the instrument reads 80 was found to be 10.7 per cent instead of 
15.7 per cent, as above, then, since the instrument reading was 1 per 
cent too low, at 90 it would be 0.5 per cent too low, at 95, 0.25 per cent 
too low, and so on. A similar determination should be made above the 
100-point position to determine the departure on that side. 

ELECTRICAL METHOD OF DETERMINING TEMPERATURE. 

The change in the electrical resistance of metals with temperature 
enables us to employ this property as a thermometer by measuring 
variations in the electrical resistance of a suitable coil of wire located 
at the desired point. This method has the very important advantage 
of enabling the temperatures in very inaccessible places to be deter- 
mined, owing to the fact that it is only necessary to have a small tem- 
perature coil at the point where it is desired to make the measurements, 
while the measuring instrument can be located at any convenient place, 
connection with the temperature coil being made by means of wires. 
A convenient instrument for this purpose has been devised and lends 
itself readily to the measurement of temperatures in inaccessible places, 
such as the temperature of the soil at various depths, the temperature 
of fermenting tobacco heaps, of rooms, of tanks, and, in fact, at any 
point where it is desired to determine the temperature and where it is 
inconvenient to use mercurial thermometers. 

ELECTRICAL THERMOMETER. 

This instrument is of the same dimensions and general construction 
as the soil hygrometer, the essential difference between the two instru- 



28 

ments being that in the soil hygrometer two arms of the bridge — namely, 
the soil resistance and the temperature cell — are outside the instrument, 
while in the electrical tbermometei it is necessary to have only one arm 
of the bridge — namely, the temperature coil — outside the instrument. 
In other words, the compensating cell in the soil hygrometer has been 
supplanted in the electrical thermometer by the comparison coil within 
the instrument. The condenser is omitted, since the slight capacity of 
the temperature and comparison coils are practically equal and no 
disturbing effect is noticed. The comparison coil is made of manganin 
wire, the resistance-temperature coefficient of which is practically zero 
at ordinary temperatures. 

Different metals vary in the change in electrical resistance which 
they experience with the change in temperature. In order to make the 
thermometer sensitive it is important to use a metal having a high 
temperature coefficient. For this purpose iron has been selected, as it 
has the greatest temperature coefficient of any of the common metals 
and also possesses the advantage of a relatively high specific resist- 
ance, thus necessitatiug the use of only a small quantity of wire and 
enabling temperature changes to be taken up more quickly. The wire 
in the temperature coil is of No. 36 B. and S. gauge, drawn from iron as 
free from carbon as possible, and is covered with single cotton insula- 
tion. Since the temperature coefficient of iron seems to vary greatly 
with the amount of carbon present, it was deemed necessary to deter- 
mine the temperature coefficient. This was done by making an open 
coil of a suitable length of the wire, providing it with heavy copper 
lead- wires, and placing the whole in an oil bath which was gradually 
heated up to 160° F. and then allowed to cool slowly, the resistance of 
the wire being noted at 5 degree intervals as the bath cooled. The 
variation in resistance with tbe temperature, computed from the curves 
actually obtained, is given in the following table and is shown graph- 
ically in the accompanying curve: 

Calibration table for electrical thermometer. 



Tempera- 
ture 
(degrees F.). 


Resistance 
. (ohms). 





77.8 


10 


80.6 


20 


83.3 


30 


86.0 


40 


88.7 


50 


91.5 


60 


94.3 


70 


97.1 


80 


100.0 


90 


102.9 


100 


105.9 


110 


108.9 


120 


112.0 


130 


115.2 


140 


118.4 


150 


121.5 


160 


124.7 



29 



The resistance-temperature curve of the iron wire used iu the tem- 
perature coils having been determined, the resistance of the temper- 
ature coil for every 10 degrees throughout the desired range can be 
found from the curve. The temperature instrument can then be cali- 
brated by balancing the instrument against the resistances correspond- 
ing to these temperatures. The usual range of temperature for each 
instrument is 100 degrees on the Fahrenheit scale, which may be 
selected from any part of the absolute temperature scale up to temper- 
atures which would destroy the temperature coil. This can be accom- 
plished by adjusting either the resistance of the comparison coil or the 
temperature coils. The former method is preferable, since the temper- 
ature coils can then be used in connection with any instrument. 

Owing to the fact that the variations in the resistance in the temper- 



O 10 20 30 <tO 50 GO 70 SO 00 100 110 120 130 140 J50_J6Q 



120 



Sp 110 



Uj 100 



90 



so 



70 



_^r: . 



120 



no 



mo 



90 



80 



70 



O 10 20 30 40 50 60 70 HO 90 100 110 120 130 1-1-0 150 160 

TEMPERATURE— DEGREES FAHRENHEIT, 
Fig. 9.— Resistance-temperature curve for the iron wire used iu the temperature coils. 

ature coil are slight, as compared with the variations in the resistance 
between the soil electrodes, it is necessary to increase considerably the 
length of the bridge wire of the electrical thermometer in order to 
secure an open scale on the instrument. This is accomplished by add- 
ing to the bridge wire small coils of insulated platinoid wire. The 
sensibility, and consequently the range of the instrument, can be varied 
by changing the resistance of these coils. Any change, however, either 
in the resistance of these bridge-wire coils or of the temperature coil 
necessitates, of course, the construction of a new scale. 

TEMPERATURE COILS. 

The temperature coils are all adjusted to have a resistance of 100 
ohms at 80° F. The coils are prepared by taking a suitable length of 



30 

wire (about 40 feet), doubling it to diminish induction effects, and then 
forming it into a narrow and rather loose coil about 8 inches in length. 
The coil is then adjusted to the required resistance and slipped into a 
piece of lead tubing about three-eighths of an inch in diameter, which 
is sealed at one end. The terminals of the coil are next soldered to the 
two wires of a two-conductor lead-covered cable, and the lead tubing 
which surrounds the coil is soldered to the lead covering of this cable. 
The coil is now thoroughly x^rotected from moisture with the exception 
of a possible leakage through the lead-covered cable, which is pre- 
vented by immersing the free end in boiling paraffine until the air has 
been displaced. As an additional precaution it has been customary 
before putting the coil into its leaden sheath to saturate it with insu- 
lating oil. 

The two conductor cable to which the temperature coil is attached 
should be about 8 inches long. The insulated conductors project 4 
inches beyond the end of the lead sheath, and to these terminals insu- 
lated wires of sufficient length to connect to the measuring instrument 
are carefully spliced (see p. 25). The spliced portious are then thor- 
oughly covered with insulating wax and thoroughly taped, as it is very 
important that there should be no leakage at these points. When 
possible the spliced portions should be soldered. 

When the connecting wires are each not more than 20 feet in length, 
No. 10 B. and S. copper wire may be used. If not more than 100 feet 
in length, No. 12 B. and S. wire gauge will be suitable for the connec- 
tion. The error introduced by the resistance of the connecting wires 
of the dimensions given above will always be less than 1°F. More 
accurate results can be obtained by using larger wires. 

USE OF THE ELECTRICAL THERMOMETER. 

The electrical thermometer is operated in exactly the same way as the 
soil hygrometer. The terminals of the temperature coils are connected 
to the two binding posts of the instrument. The handle of the instru- 
ment is then depressed, when a buzzing note will be heard in the tele- 
phone receiver, which must be held close to the ear. Pressing the 
handle down firmly it is rotated to the right or left until the point is 
reached at which the sound in the receiver disappears or grows very 
faint. A little difficulty may be experienced at first in locating the 
exact position of the pointer for this minimum sound. It is of assist- 
ance in such a case to move the pointer rapidly back and forth over the 
point at which the sound disappears, noting the point at which the 
sound begins to increase on either side. The mean position between 
these two points gives the desired reading. 

LOCATION OF FAULTS. 

Owing to the similarity in construction of the hygrometer and the 
thermometer the. directions given for the location and correction of 



31 



faults in the former instrument will apply equally well to the latter. 
The thermometer differs from the hygrometer only in the addition of a 
comparison coil, the connections for which are seen in the accompany- 
ing diagrammatic sketch of the instrument (fig. 10). 



ADJUSTMENT OF INSTRUMENT. 



The adjustment of the thermometer is easily tested by comparing 
some temperature, as determined by the temperature coil, with that 



WIF?£ 



BRIDGE WIRE 
CONTACT 



TO 
TELEPHONE 
RECEIVER 




COMPARISON 
COIL 



BINDING POST 



INDUCTION COIL 



BATTERY 



BATTERY SWITCH 

Fig. 10. — Diagram of interior connections of electrical thermometer. 

given by a good mercury thermometer. Place a temperature coil in a 
large bucket of water at about room temperature, allow it to remain 
for several minutes to acquire the temperature of the water, and then, 
keeping it immersed, connect with the instrument and determine the 
temperature. At the same time the temperature of the water, which 
must be frequently stirred, should be determined by means of a relia- 
ble mercury thermometer. Should a discrepancy of more than one 
degree occur between the two determinations the test should be 



32 

repeated. It is also advisable to use several different temperature 
coils, as the difference might be due to an error in the adjustment of 
the temperature coil itself. If a practically constant discrepancy 
should be found between the readings of the mercury thermometer and 
tbe temperature as determined by the temperature coils, then the 
adjustment of t lie instrument is incorrect and the set screw holding 
the pointer should be loosened and the pointer adjusted until the read- 
ings of the instrument and the mercury thermometer agree. In case 
it is necessary for any reason to remove the shaft and the contact arm 
of the balancing mechanism, this adjustment should always be made 
on reassembling the instrument. 

STANDARDIZATION OF TEMPERATURE COILS. 

Although it is the intention to test each temperature coil carefully 
before it leaves the laboratory, it sometimes happens that the coils sub- 
sequently develop faults or are found not to be perfectly adjusted. It 
is therefore recommended that the various coils be compared with a 
good thermometer and with each other in the manner just described. 
In this way the corrections to be applied to the reading of each coil 
may be determined, and may or may not be used, according to the 
degree of accuracy desired. 

APPARATUS FOR DETERMINING THE SALT CONTENT OF SOILS. 
ELECTROLYTIC BRIDGE. 

By slightly modifying the electrical thermometer the instrument can 
be adapted to the determination of the salt content of soils. 1 In fact, 
the instrument as used for this purpose is nothing more or less than a 
slide- wire bridge adapted to the measurement of electrolytic resistances. 
It is also equally applicable to the measurement of nonelectrolytic 
resistances. The instrument differs from the electrical thermometer 
only in having the resistance of the comparison coil in the third arm 
adjustable, in order to adapt the instrument to a greater range of meas- 
urements. 

ROTARY SWITCH. 

The adjustment of the Comparison coil is accomplished through the 
agency of a small rotary switch operated from the outside of the box, 
by means of which 10, 100, or 1,000 ohms can be introduced as the 
third arm of the bridge. This is accomplished by joining in series a 
10, 90, and 900 ohm coil connected to the three segments of the switch 
in such a way that the 10-ohm coil can be thrown in alone, or the 10-ohm 
and 90-ohm coils in series, or all three coils in series. The manner of 

1 For an account of this method see article by Thos. H. Means on "A Rapid Method 
for the Determination of the amount of Soluble Mineral Matter in a Soil," Am. Jour, 
of Science, Vol. 7, 1899, p. 264. 



33 

connecting the coils, together with the general connection of the instru- 
ment, is shown diagraniatically in fig. 11. 

The segments of the rotary switch are mounted upon hard rubber 
and the spring through which contact is made with the several seg- 
ments is insulated from the shaft by which it is operated, in order to 
avoid stray circuits from the binding posts to the switch when using 
the instrument in the held. 

All contact surfaces are heavily nickeled, in order to prevent oxida- 



WIRE 



BRIDGE WIRE 
CONTACT 



TO 
TELEPHONE 
RECEIVER^ 




BINDING POST 



BATTERY 



Fig. 11. — Diagram of interior connections of electrolytic bridge. 

tion. The shaft operating the switch carries, just below the handle, a 
beveled collar upon which are three graduations, numbered 10, 100, and 
1,000. These various resistances can be successively thrown in as the 
third arm of the bridge by simply bringing the corresponding gradua- 
tion to coincide with an index mark upon the bushing beneath. By 
thus having the graduations upon the movable collar instead of upon 
a stationary scale, the index mark can be chosen in the position most 
1980S— Xo. 15 3 



34 

convenient for reading, so that the resistance of the comparison coil 
can be determined at a glance. 

USE OF THE ELECTROLYTIC BRIDGE. 

The instrument is operated in a manner similar to the hygrometer 
and thermometer. The resistance to be measured is connected with 
the two binding posts of the instrument. The handle is then pushed 
down and rotated until a point is found at which the sound in the 



^-Q \ 




Fig. 12. — Electrolytic cell and mercury cups. 

receiver disappears. In case a balance is not obtained with the 1,000- 
ohm coil thrown in, the other coils should be tried. It is always best 
to choose the coil which will bring the balance as near as possible to 
the center of the scale, as this is the most sensitive position. 

The balance obtained, the resistance is found by multiplying the 
resistance of the comparison coil, as shown by the rotary switch, by the 
number on the scale opposite the pointer. Thus, if the comparison coil 
is 100 ohms and the reading on the scale is 0.92, the resistance between 
the posts is 92 ohms; if the comparison coil is 1,000 ohms and the read- 
ing on the scale is 8.5, the resistance would be 8,500 ohms, and so on. 
As the scale is graduated from 0.4 to 10, the instrument has a range of 
from 4 ohms to 10,000 ohms. 



35 

ELECTROLYTIC CELL. 

The salt determinations are made in an electrolytic cell Laving a 
capacity of about 50 c. c. The cell and the mercury cups by which it is 
connected to the instrument are illustrated in fig. 12. The cell is con- 
structed of hard rubber, with brass electrodes, which are heavily nickel 
plated but not burnished, in order to give greater surface area to the 
electrodes and to afford better contact with the moist soil. It is cylin- 
drical inside with a cup-shaped depression in the base, this form greatly 
facilitating the removal of soil. 

The mercury cups are so arranged as to be capable of being swung 
over the instrument during transportation, rendering the arrangement 
more compact. The mercury is retained in the cups when the instru- 
ment is being carried about by means of two flat springs fitting over 
the tops of the cups. 

LOCATION OF FAULTS. 

The methods of locating any faults which may occur in the instru- 
ment are similar to those used in the hygrometer and thermometer. For 
purposes of standardization and adjustment a 100-ohm coil will be fur- 
nished with each instrument. When connected between the two bind- 
ing posts the bridge reading should of course be 1.0, if balanced against 
the 100-ohm coil. This test of the instrument should occasionally be 
made to note the adjustment. In case readjustment should be neces- 
sary, the screw holding the pointer is loosened and the pointer adjusted 
in the manner already described for the hygrometer. 



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